This paper describes a new method to measure the shunt resistance Rp of each solar cell in installed PV modules. The shunt resistance Rp determines the power output of the PV module especially at low irradiance and is therefore crucial for calculating a reliable power forecast as well as for evaluating the effects of degradation and ageing particularly if the module is affected by Potential Induced Degradation (PID). Our method uses two daylight photoluminescence (dPL) images captured with the DaySy Tool under daylight conditions at two different solar irradiances levels 1 and 2. The method is applied to a healthy and a PID affected module: The healthy module shows a mean Rp = 5710 ?/cm2 with negligible power loss at low irradiances. However the PID module shows a heavily reduced mean Rp = 588 ?/cm2 leading to a power loss of -42 % at = 100 W/m2 compared to the healthy module. Hence our method reflects the expected behavior of PID affected modules.
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